Squelch circuit

ABSTRACT

A squelch circuit in which a phase locked loop (PLL) (5) having a voltage controlled oscillator (VCO) (7) and a phase comparator (10) is used to detect the number of zero crossings, these being low for a speech signal and high for noise. The error voltage produced in the phase locked loop (5) is fed through a network (12) which ensures that the matching of the input signal to the oscillator is faster when the input frequency drops than when the input frequency rises. The input signal is clipped to enhance the frequency spectra of speech by using a limiting amplifier. The muting gate (2) output is derived from the VCO (7) control voltage of the PLL (5) to mute the output of the receiver when the VCO (7) output frequency is higher than a pre-set frequency for more than a pre-set time.

This invention relates to the processing of audio signals in relation toderiving a squelch function for use with radio receivers. The term`squelch` refers to the muting of or significant reduction of the levelof audio output of a receiver when there is no intelligence beingreceived, thereby relieving the aural strain of an operator why may haveto maintain a listening watch for long periods.

A variety of solutions to this requirement have been proposed.Australian Pat. Nos. 500,961 and 509,953 each deal with a squelchfacility to reduce background noise by comparison of energy levels inthe frequency range outside the audio range. Other techniques which havebeen used are:

1. Comparative assessment of energy levels present in narrow sub-bandsof the received audio band;

2. Real-time analysis of the audio signal;

3. The use of coded preambles and postambles on the transmitted signal;

4. The use of a tone transmitted continuously with the signal; and

5. Real-time analysis of the zero-crossings present in the audio signal.

The technique described in the U.S. Pat. No. 3,939,425 takes its inputfrom the intermediate frequency (IF) signal of the receiver, to derivethe muting signal. It also demodulates the IF signal to recover theaudio output for the receiver.

It appears, from the description, that the technique is applicable onlyto amplitude modulated (AM) signals. Thus, the technique requires accessto a signal (the IF signal) which is not normally available as areceiver output, and is applicable only to AM signals.

The above approaches have varying degrees of complexity and performtheir function with varying degrees of success. In contrast thetechnique which is the subject of this invention has the object of beingsimple to implement, while providing excellent performance, and uses asits input only the audio signal from the receiver, thus not requiringaccess to a receiver IF signal.

To this end the present invention provides a squelch circuit for mutingaudio output of a receiver except when a desired audio signal isreceived, comprising: an input terminal connected to a limitingamplifier and a muting gate; a phase locked loop which receives thelimiting amplifier output and which contains an oscillator and a phasecomparator for comparing the frequency of said oscillator and saidlimiting amplifier output to produce an error voltage which is fed backto control said oscillator through a network to enable the oscillator tomatch its frequency to that of the limiting amplifier output, the matchoccurring faster when the limiting amplifier output frequency drops thanwhen the limiting amplifier output frequency rises; said muting gatebeing connected to the output of said phase locked loop to mute theoutput of the receiver when the limiting amplifier output frequency ishigher than that desired.

The device of this invention performs an analysis of a receiver audiooutput in terms of the zero-crossings of the signal and provides anoutput signal which can be used to control the muting of the system.

Examination of the spectra of audio signals produced by a radio receivershows that for speech, the frequency of the dominant spectral componentslies in the range from about 100 Hz to 300 Hz for male voices and fromabout 200 Hz to 700 Hz for female voices. These figures hold true forthe majority of english speech, the exception being with sibilants andfricatives. With noise, however, the frequency of the dominant spectralcomponents covers a much wider range, limited by the band-width of thereceiver.

Preferably the oscillator in the phase locked loop is a voltagecontrolled oscillator (V.C.O.). The muting gate output is preferablyderived from the V.C.O. control voltage of the phase locked loop to mutethe output of the receiver when the V.C.O. output frequency is higherthan a pre-set frequency for more than a pre-set time.

A preferred embodiment of this invention will now be described withreference to the circuit diagram shown in the FIGURE.

The audio input is shown at 1 a muting gate at 2 and the output at 3.

The present embodiment uses a limiting amplifier 4 to process the audiosignal, and uses a phase-locked loop (PLL) 5 to determine the averagezero-crossing rate of the limited signal.

If the audio output from a receiver is passed through a limitingamplifier, thus removing all information that is not near the center ofthe waveform, the resultant spectrum has the dominant spectra enhanced,thus providing a low (100 to 700 Hz) average zero-crossing rate forspeech and a higher (800 to 3000 Hz) average zero-crossing rate fornoise. The PLL 5 attempts to track the average frequency of the clippedaudio signal.

The PLL device contains a voltage-controlled oscillator (VCO) 7, aninput amplifier 8, a type I phase comparator 9, a type II phasecomparator 10, a source follower 11, and a zener diode 11a. The VCOsignal from 7 is compared in frequency with the clipped audio signal bythe phase comparator 10 and an error voltage derived. The type I phasecomparator 9, the source follower 11 and the zener diode 11a all ofwhich are components of this standard PLL component, are not used inthis application of the invention.

The feedback provided by the error voltage generated by the phasecomparator 10 attempts to maintain the VCO frequency equal to theinstantaneous frequency of the clipped audio. Thus, the VCO controlvoltage corresponds to the average zero-crossing rate of the inputsignal. The error voltage is fed through a network 12 which providesfast attack when the input frequency drops, and a slower decay when thefrequency rises.

Thus, when the average zero-crossing rate suddenly reduces,corresponding to the appearance of speech components on the receiversignal, the VCO frequency will quickly attempt to follow, rapidlylowering the VCO control voltage which is used to unmute the receiver.If the zero-crossing rate later increases, as would occur at the end ofa transmission when the carrier ceases, the VCO frequency will riseslowly, and will only mute the receiver when it has risen above aspecific threshold voltage. This fast attack and slow decay allows themuting to be held off between words of the speech, even though thezero-crossing rate is high during this time.

The VCO control voltage is fed to a comparator 13 to derive the mutingsignal. When the VCO control voltage is lower than the comparatorreference (half the supply in this case) the muting signal is high. Whenthe VCO control voltage is greater than the reference, the muting signalis low. The comparator signal is further smoothed and modified in itsattack and decay characteristics by a non-linear filter 14 on thecomparator output, and fed to a muting switch 2 in the form of ananalogue gate for the control of the audio output signal. The output ofthe comparator is also fed to a light emitting diode 15 which givesvisual indication that the VCO frequency is below the specified limit,corresponding to the detection of speech.

Since the spectral nature of speech and the noise output of a receiverboth vary, an adjustment 16 is provided to set the optimum centrefrequency and tuning range of the VCO. The adjustment of the frequencycharacteristics of the VCO is implemented in a manner somewhat differentto that generally used, and provides an essentially constant ratiobetween the centre frequency and the tuning range of the VCO as thecentre frequency is adjusted. It has been demonstrated in testsinvolving a wide range of speech and noise spectra that the constantratio between centre frequency and tuning range has advantage over theusual characteristic of the VCO where this ratio increases significantlywith centre frequency.

With strong signals, the centre frequency setting is not critical, butto detect speech in very poor signal/noise ratios, the optimumadjustment range is narrower.

Because of the finite response time of the signal processing, unmutingwill occur slightly after speech is first received, resulting in thepartial loss of the first syllable of speech. In this embodiment, thereaction time of the system is such that very little degradation resultsfrom the delay. However, should this degradation be unacceptable, it canbe overcome by incorporating a suitable delay in the path between theaudio input and the muting switch.

The claims defining the invention are as follows

I claim:
 1. A squelch circuit for muting audio output of a receiver except when a desired audio signal is received, comprising: an input terminal connected to a limiting amplifier and a muting gate; a phase locked loop which receives the limiting amplifier output and which contains an oscillator and a phase comparator for comparing the frequency of said oscillator and said limiting amplifier output to produce an error voltage which is fed back to control said oscillator through a network to enable the oscillator to match its frequency to that of the limiting amplifier output, the match occurring faster when the limiting amplifier output frequency drops than when the limiting amplifier output frequency rises; said muting gate being coupled to an output of said network to mute the output of the receiver when the limiting amplifier output frequency is higher than that desired.
 2. A squelch circuit as claimed in claim 1 in which the limiting amplifier enchances the dominant frequency spectra of speech. 